I purchased a rider on my commercial liability policy that extends rental car coverage to unlimited days and covers anything not covered by other insurance, including my VISA coverage. It is about $100 a year and I get to hand any claims off to my agent at the local insurance company that I use. National does not want to have to negotiate with Janet. Janet takes care of her customers.
I looked at the AMEX coverage, and it would be a pretty good deal for me as most of my rentals are 3-4 days compared to the rental car company rates.